This invention relates to a simple device and procedure for accurately setting up heavy capacity scales (such as truck and railway track scales) and for subsequently checking and readjusting such machines in order to maintain their accuracy of weight determination.
Normally in setting up or calibrating any scale installation dead weights of known certified weight are used. For scales of heavy capacity (truck and track scales of 100,000 lbs plus capacity) it is common practice to use known test weights amounting only to a fraction weighwise of the scale capacity--20,000 lbs, for example, in the case of truck scales. After the scale under test has been adjusted to indicate correctly the test weight value, substitute dead weight material (of about the same weight as the test weight) is placed on the scale and the test weight is used as an addition to set up the scale further up its capacity range. This procedure is repeated until the scale has been set up or calibrated all the way up to its full capacity. This procedure is not only used on calibration of new installations but is also used to periodically check all scales which are in service.
This substitution procedure method of scale calibration has some inherent disadvantages and limitations which are listed below as follows:
(1) The handling (cranes are required), transportation and maintenance of 20,000 lbs of certified test weights is a very costly item with modern test trucks costing in excess of $100,000.00. Furthermore with modern truck scales going up in capacity (it is not uncommon to find truck scales rated 100 tons) 20,000 lbs of test weights is generally thought to be insufficient to do a proper scale calibration. In testing track scales it becomes necessary to provide specialty test cars in additiont to having at least 20,000 lbs of certified test weights. PA1 (2) The dead weight substitution material is usually difficult to obtain at most sites particularily in weight values approaching the ideal 20,000 lbs increments. There is also at least one increment of uncertainty in the substituted material weight value unless great care is taken to establish the weight to the nearest increment of resolution. PA1 (3) The procedure of handling test weights and substituting dead weight material is quite time consuming and the problem arises of being able to differentiate between errors of weighing and normal zero movement due to temperature changes, etc which occur on all electronic scales with the elapse of time.